1. Field of the Invention
The present invention relates to a mounting apparatus, and in particular, to a mounting apparatus for a motor used in a centrifuge instrument.
2. DESCRIPTION OF THE PRIOR ART
In a centrifuge instrument the rotating member, or rotor, forms part of a system that includes a motor or other source of motive energy a drive shaft, and a rotor mounting device called a spud disposed at the upper end of the shaft on which the rotor is received. It is advantageous for a variety of reasons to cause the rotor to rotate with its center of gravity as close as possible to the axis of rotation. Initially, upon startup of the instrument the mass of the rotor has a tendency to spin on its geometric center. However, as rotor speed increases there occurs a shift in which the mass of the rotor spins about its center of gravity. The speed at which this shift occurs is known as the critical speed of the rotor. Violent motion and vibration are imparted to the rotox and the drive as it reaches the critical speed. However once the critical speed is reached the vibration is significantly decreased. Typically the drive is provided with some form of compliance mechanism which accommodates the forces imposed on the system as the rotor approaches and passes through its critical speed.
Historically centrifuge drives have developed along two distinct paths related to the use of such drives in different centrifuge rotational speed regimes. The drives for so-called lower speed centrifuges (i.e., those having a speed less than twenty thousand revolutions per minute) typically use rigid shafts that are either directly coupled to a drive motor or are belt driven. Any compliance required for the stable operation of the instrument is achieved by the use of elastomeric "shock" mounts. Design of drives for higher speed instruments using such mounts is difficult since the dynamics of the system is influenced by the entire mass supported by the shock mounts, and not merely the rotor mass.
In higher speed centrifuges such as those used in the so-called ultra speed range (i.e., above twenty thousand revolutions per minute) the compliance problem is solved, but at the expense of simplicity, ruggedness and cost, by reducing the dynamic mass and allowing compliance to take place through the flexure of a relatively long and delicate shaft on which the rotor is mounted.
Simple shock mounts cannot be used at higher speeds because of their low lateral and torsional stiffness compared to their pivot or moment stiffness. The moment stiffness is dependent on compression or extension of the elastic shock mount whereas the lateral and torsional stiffness are determined by shear of the elastic mount. For a given mount configuration the shear stiffness is usually only one third of the compressive stiffness. For this reason it is difficult to design critical speed out of the operating range from these three vibration modes.
U.S. Pat. No. 4,511,350 (Romanauskas) relates to a suspension system for a centrifuge. Additionally, there is known a flexural pivot system sold by Bendix Aerospace in which springs are used to accommodate forces.
In view of the foregoing it is believed to be desirable to provide a centrifuge drive that is able to allow significantly higher operating speed with simpler, more rugged, rigid shaft design. It is also believed advantageous to provide a drive having a motor mount that exhibits a relatively high lateral, vertical and torsional stiffness relative to the moment stiffness.